Chronic pain is one of the most common complaints of patients who undergo surgical procedures or suffer from long term illness, including cancer, nerve injury, athritis, or heart disease. Pharmacological agents used for treating chronic pain are often unsatisfactory because of the side effects accompanied the treatment. For example, opiates, at high doses, are apt to produce sedation, respiratory depression and tolerance, which severely limit their use. Recently, genetic approaches have been attempted to manage the chronic pain. One popular strategy is to increase the production of endogenous μ-opioid receptor ligands by introducing opioid precursor genes for enkephalin and β-endorphin into DRG neurons or meninges surrounding the spinal cord through adeno- or herpes viral vectors. The enhancement of opioid peptides effectively reduces nociceptive behaviors in rats. The shortcomings of this approach are transient expression of the target genes and potential possibilities of tolerance development as result of upregulation of opioids.
Thus, there remains a need for method for managing chronic pain. The present invention, at least in part, is directed to meeting this need.